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This invention relates generally to a traffic management system for use in a network-like, vehicle-based, automated material handling system and in particular to a traffic management system that assigns priorities to jobs with a system and method that uses node traffic balancing and node traffic density values.
Electrically powered motor transport vehicles (MTVs) are often used in manufacturing and warehouse environments for transporting and manipulating articles of manufacture. Such vehicles are desirable in such environments due to their clean operation and low noise. Often such vehicles are propelled along a fixed rail or track, allowing precise control of movement along a predetermined path.
In particular, computer controlled materials transport systems are known for moving materials among various work stations of a facility. Such systems are employed, as an example, in semiconductor fabrication facilities for moving semiconductor wafers to successive work stations. In such a wafer transport system, a monorail track is routed past the work stations and a plurality of electric vehicles are mounted on the track and moveable therealong for delivering wafers to successive work stations and for removing wafers therefrom after requisite processing operations have been accomplished. The track is composed of interconnected track sections that usually include one or more routing sections or modules that are operative to provide plural paths along the track. In general a node is a location where a vehicle is stopped, loaded, unloaded, or redirected. Thus, a node can be a workstation that a vehicle must pass through or an intersection of one or more tracks where the vehicle may be redirected.
The vehicles on the track typically operate in a connected mode. In the connected mode of operation, a central controller, usually a computer, assigns destinations to vehicles and monitors the operation of the entire material handling system. This monitoring may include monitoring the status and location of each MTV, the status and location of material lots needed to be transported, and the status of each node of the material transport system.
The central controller is therefore responsible for the execution of move requests to transport a material lot from a source node to a destination node. In addition, the central controller is also responsible for the overall efficiency of the material transport system. For example, the more efficient use of the MTVs results in needing fewer of them, which reduces the cost of the overall system. In addition, by avoiding xe2x80x9ctraffic jamsxe2x80x9d at busy nodes the central controller may avoid a number of MTVs waiting in a queue for a pick-up or a delivery of a material lot. While waiting in a queue the MTV is being used inefficiently and the material transport system will experience longer delivery times.
Prior art systems provide a list of material lot move requests that are scheduled and prioritized by a first-in-first-out (FIFO) methodology. Each move request that is received includes a time-stamp associated with it that indicates when the move request originated. This method, however, allows move requests having a more recent time-stamp to be executed before a move request having a more stale time-stamp. This can result in an increase in delivery time and a decrease in the overall efficiency of the system. In addition, other problems may be caused by the use of a FIFO schedule/prioritizing scheme. For example, traffic jams occur at busy nodes resulting in an increased backlog of unexecuted move requests, longer delivery times, and MTVs waiting in queues to pick or drop off a lot.
Therefore, it would be desirable to provide a system and method that increases the efficient use of the MTVs, reduces the average delivery time of loads to stations, and reduces congestion at load/unload nodes.
A system and method for prioritizing the execution of move requests in an automatic material handling system includes providing an ordered list of move requests to a material control system, wherein the ordered list includes a source node and a destination node. The material control system calculates a node traffic balance value and a node traffic density value for each source node and each destination node corresponding to each move request. The material control system searches the ordered list of move requests for the first move request having a source node and destination node that have a traffic balance value equal to a predetermined first traffic balance value and a traffic density value equal to a predetermined first traffic density value. The material control system assigns the first move request having these parameters a first priority value.
The material control system then searches the remaining move requests in the ordered list of move requests for a second move request having an associated source node and destination node that have a traffic balance value equal to a predetermined second traffic balance value and a traffic density value equal to a predetermined second traffic density value. The material control system assigns this second move request a second priority value.
The material control system searches the remaining move requests in the ordered list of move requests for a third move request having a source node that has a free car request value equal to a predetermined first free car request value. The material control system assigns the third move request a third priority value.
Additional aspects, features and advantages of the present invention are also described in the following Detailed Description.